In the manufacture of many types of paper, mineral fillers are incorporated with the pulp to improve such properties as color, opacity and printability. The fillers also allow the use of lessened amounts of cellulosic fiber in the pulp. Typical levels of filler addition in acid sized papers range from about 5 to about 12% by weight; but more recent alkaline sized papers can approach filler levels of 20% or more, particularly if expensive retention aids are included. However, large proportions of the fillers added to the pulp normally pass through the paper-making screens and are not retained in the paper web.
There have been many attempts to use low cost calcium sulfate fillers in paper making as a partial replacement of the paper pulp. Most of those attempts have centered around the use of finely ground gypsum (calcium sulfate dihydrate) or anhydrite spheroidal particles.
These relatively round particulate fillers are not at all well retained in the paper web. The use of retention aids helps but at a significant cost penalty. Specially prepared very elongated crystalline fiber forms of calcium sulfate having very high aspect ratios greater than 50:1 or even greater than 100:1 have been proposed for use in paper. These are said to be well retained without the use of retention aids but their preparation methods make them very expensive. They may be prohibitively expensive for most coarse papers, liners and paper boards, and their use may be generally confined to only the most refined grades of fine papers.